This disclosure relates to an engine control system and method used to compensate for transient EGR error in a Gasoline Direct-Injection Compression-Ignition (GDCI) engine.
Exhaust gas recirculation (EGR) using a low-pressure loop (LPL) is increasingly used for turbocharged spark-ignited engines and compression ignited engines. The LPL supplies exhaust gases downstream from the turbocharger back to the engine's combustion chambers. By way of contrast, a high-pressure loop (HPL) would recirculate exhaust gases back to the combustion chambers from a location upstream of the turbocharger.
LPL EGR is typically preferred over HPL EGR for several reasons, including, a) there is less compromise of the boost system since EGR gases are drawn after the turbocharger, b) the exhaust gases can be cleaned in a catalyst to reduce soot and hydrocarbons that may otherwise foul the engine air system, and c) the exhaust gases are cooler and closer to target charge temperatures with reduced cooler heat load. However. LPL EGR has a disadvantage for engine response time in that LPL EGR systems have relatively long ducts between the EGR valve and the intake ports to the combustion chambers. This causes greater delay to deliver EGR to the cylinders during load transients.
EGR transport delay is defined as the time required for EGR gases to flow from the EGR valve to the intake valves. EGR transport delay causes the EGR to deviate during transients from the calibrated EGR level determined in steady state dynamometer tests. EGR errors compromise transient performance because combustion phasing depends on the amount of EGR in the cylinder. Excessively high amounts of EGR produce longer ignition delay, retarded combustion phasing, and potentially misfires. Too low amounts of EGR cause shorter ignition delay, advanced combustion phasing, and may lead to excessive combustion noise. Deviations from the calibrated, steady state EGR level will also cause compromises in fuel consumption and emissions.
Inert EGR is defined as the burned species portion of the EGR gases, not including the unburned air portion of the EGR. During a fast load increase (“tip in”), it is apparent that inert EGR is lower than targets and inert EGR error may be large. Similarly, for a fast load decrease (“tip out”). EGR transport delay causes higher than desired EGR. This is because the EGR in the EGR and air systems is higher before the tip out than what is demanded after the tip out. The EGR must purge out prior to reaching lower targeted levels. The time difference between desired and actual EGR is an estimate of EGR transport delay. EGR transport delay for some GDCI engines may be in the range of 0.5 to 0.7 sec.
While the EGR and air systems are designed to be compact with the least possible volume and duct length, there will always be some amount of EGR transport delay that causes deviation during fast transients from the calibrated steady state values. What is needed is a system and method to compensate for EGR errors during momentary fast transients.